Abstract

When thinking of renovating the windows of old buildings, the building owner has a lot of
decisions to make. These are to keep the window but make it more energy efficient by adding
an extra pane or to completely change the whole window. At the same time, the joint between
the window frame and wall makes a thermal bridge which depends on how much insulation
has been placed in the cavities after installation. Upon the decision of keeping the window,
the status of this joint will be unchanged. This thesis deals with finding out the best economical
solution for a company that has rental apartments in Gävle in Sweden, Gavlegårdarna AB. To
calculate the thermal bridges, which are weak areas of the building envelope in which they
significantly increase the energy load of houses, a CDF program called Fluent is used to
quantify the heat loss at the joints. Measurements have been performed to validate the CFD
model. To simulate the energy savings in the building, the building energy simulation program
IDA-ICE is used. Finally, Life Cycle Costing calculations are made to assess the best long term
economical option. It is concluded that the most reasonable solution is to add an extra glass
in the existing window, but it is not the most ecofriendly. A more ecological solution is to add
the extra glass and to perform enhanced insulation at the joints around the window frames
and walls, or to replace the old window with a new low energy window – however, these are
not profitable solutions.